Rayleigh (of Terling Place), John William Strutt, 3rd Baron ( born Nov. 12, 1842 , Langford Grove, Maldon, Essex, Eng.—died June 30, 1919 , Terling Place, Witham, Essex ) English physical scientist who made fundamental discoveries in the fields of acoustics and optics that are basic to the theory of wave propagation in fluids. He received the Nobel Prize for Physics in 1904 for his successful isolation of argon, an inert atmospheric gas.

Strutt suffered from poor health throughout his childhood and youth, and it was necessary for him to be withdrawn from both Eton and Harrow. In 1857 he began four years of private study under a tutor. In 1861 Strutt entered Trinity College, Cambridge, from which he was graduated with a B.A. in 1865. He early developed an absorbing interest in both the experimental and mathematical sides of physical science, and in 1868 he purchased an outfit of scientific apparatus for independent research. In his first paper, published in 1869, he gave a lucid exposition of some aspects of the electromagnetic theory of James Clerk Maxwell, the Scottish physicist, in terms of analogies that the average man would understand.

An attack of rheumatic fever shortly after his marriage in 1871 threatened his life for a time. A recuperative trip to Egypt was suggested, and Strutt took his bride, Evelyn Balfour, the sister of Arthur James Balfour, on a houseboat journey up the Nile for an extended winter holiday. On this excursion he began work on his great book, The Theory of Sound, in which he examined questions of vibrations and the resonance of elastic solids and gases. The first volume appeared in 1877, followed by a second in 1878, concentrating on acoustical propagation in material media. After some revision during his lifetime and successive reprintings after his death, the work has remained the foremost monument of acoustical literature.

Shortly after returning to England he succeeded to the title of Baron Rayleigh in 1873, on the death of his father. Rayleigh then took up residence at Terling Place, where he built a laboratory adjacent to the manor house. His early papers deal with such subjects as electromagnetism, colour, acoustics, and diffraction gratings. Perhaps his most significant early work was his theory explaining the blue colour of the sky as the result of scattering of sunlight by small particles in the atmosphere. The Rayleigh scattering law, which evolved from this theory, has since become classic in the study of all kinds of wave propagation.

Rayleigh’s one excursion into academic life came in the period 1879–84, when he agreed to serve as the second Cavendish professor of experimental physics at Cambridge, in succession to James Clerk Maxwell. There Rayleigh carried out a vigorous research program on the precision determination of electrical standards. A classical series of papers, published by the Royal Society, resulted from this ambitious work. After a tenure of five years he returned to his laboratory at Terling Place, where he carried out practically all his scientific investigations.

A few months after resigning from Cambridge, Rayleigh became secretary of the Royal Society, an administrative post that, during the next 11 years, allowed considerable freedom for research.

Rayleigh’s greatest single contribution to science is generally considered to have been his discovery and isolation of argon, one of the rare gases of the atmosphere. Precision measurements of the density of gases conducted by him in the 1880s led to the interesting discovery that the density of nitrogen obtained from the atmosphere is greater by a small though definite amount than is the density of nitrogen obtained from one of its chemical compounds, such as ammonia. Excited by this anomaly and stimulated by some earlier observations of the ingenious but eccentric 18th-century scientist Henry Cavendish on the oxidation of atmospheric nitrogen, Rayleigh decided to explore the possibility that the discrepancy he had discovered resulted from the presence in the atmosphere of a hitherto undetected constituent. After a long and arduous experimental program, he finally succeeded in 1895 in isolating the gas, which was appropriately named argon, from the Greek word meaning “inactive.” Rayleigh shared the priority of the discovery with the chemist William Ramsay, who also isolated the new gas, though he began his work after Rayleigh’s publication of the original density discrepancy. Shortly before winning the Nobel Prize, Rayleigh wrote the entry on argon for the 10th edition (1902) of the Encyclopædia Britannica. (See the Britannica Classic: argon.) In 1904 Rayleigh was awarded the Nobel Prize for Physics; Ramsay received the award in chemistry for his work on argon and other inert elements. The next year Rayleigh was elected president of the Royal Society.

In his later years, when he was the foremost leader in British physics, Rayleigh served in influential advisory capacities in education and government. In 1908 he accepted the post of chancellor of the University of Cambridge, retaining this position until his death. He was also associated with the National Physical Laboratory and government committees on aviation and the treasury. Retaining his mental powers until the end, he worked on scientific papers until five days before his death, on June 30, 1919.